This invention relates generally to color picture projection systems in which a color picture from a color picture source is projected on a screen by irradiating rays of different colors, and more particularly, the present invention relates to an improvement in a lens system used in such a color picture projection system.
Customary color television sets, which are widely used, employ a cathode ray tube having a fluorescent surface to convert electrical energy into light of three different primary colors, i.e. red green and blue. However, the size of such a cathode ray tube cannot be freely increased. Therefore, in order to provide a larger picture various color picture projection systems have been previously developed, and various types of such color picture projection systems are already on market.
Generally speaking, in conventional, marketed color picture projection systems, three projection tubes are provided for respectively emitting red, green and blue light rays. These three different rays are respectively transmitted through respective lens systems to be focussed on a suitable screen so as to project a color picture image thereon by superimposing three images of different colors on each other. Therefore, the conventional color picture projection systems have required three separate lens systems, and thus, the conventional projection systems of this sort are complex in structure, resulting in a high manufacturing cost.
Here, it is to be noted that in the prior art, three different projection tubes for respectively emitting red, green and blue rays have been essential because it was impossible to provide a color picture projection system by utilizing a single cathode ray tube. To the uninitiated it would seem that an inexpensive simple color picture projection system involves magnifying the color picture on the fluorescent surface of a cathode ray tube by means of a single lens system that projects the picture on a screen. However, the diameter of each lens of such a lens system must be large as the size of the fluorescent surface of the cathode ray tube. Such a large size lens suffers from problems of chromatic aberration. The effect of chromatic aberration can be removed if various lenses are combined in a complex manner as is well known. However, the diameter of each lens in the lens system is much larger than that of easily handled cameras or the like. Such a lens system has a complex structure, is very expensive, bulky and heavy, causing the color picture projection system to have a high manufacturing cost.
For the above reason, it has been necessary for customary, practical, color projection systems to use three independent lens systems respectively responsive to light of the three primary colors emitted by three tubes.